Why the Synodontis of Lake Tanganyika all look the same

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If you have ever wondered why the Synodontis catfish of Lake Tanganyika look remarkably similar to each other in terms of colour pattern, Jeremy Wright of the University of Michigan may have the answer.

He has identified a form of mimicry rarely seen in fishes that explains this similarity in colour pattern in a study to be published in a forthcoming issue of the journal Evolution.

Biological mimicry in animals can be classified into four major forms, of which the two most common are Batesian and Müllerian mimicry. 

In Batesian mimicry, a mimic physically resembles a model but does not have the attribute the latter possesses that makes it unprofitable to predators (e.g. unpalatability); whereas in Müllerian mimicry, a number of species possessing a similar anti-predator attribute share an aposematic (or warning) coloration. 

The most widely accepted models of Müllerian mimicry evolution assume that the mimic and models are not closely related, and that they have evolved to look like each other through convergence. 

In his study, Wright shows that the Synodontis catfishes of Lake Tanganyika are Müllerian mimics of each other, and that contrary to most common models of Müllerian mimicry evolution, have evolved their warning coloration through the diversification of a common ancestor rather than through convergent evolution.

Wright carried out a series of experiments to show that the typical colour pattern of many Lake Tanganyika Synodontis catfish, which consists of numerous medium-sized black spots on a brown body and boldly marked fins, is aposematic. He did so by exposing a large, visually hunting predator (Largemouth bass, Micropterus salmoides) to individuals of S. multipunctata, S. petricola or Pimephales vigilax (the Bullhead minnow, used as a control). 

Repeating the experiment by eliminating both olfactory (by exposing conditioned bass to prey fish in a clear plastic bag) as well as other uncontrollable non-visual cues (by exposing conditioned fish to models), Wright found that the bass quickly became conditioned to avoid attacking the catfish, as well as models painted to look like the catfish.

The reason behind the reluctance to attack the catfish became clear once he studied the fin spine anatomy (revealing the presence of venom glands), as well as examined the effects of injecting the venom gland extracts into Largemouth bass (resulting in the appearance of a large dark spot originating from the injection site, loss of colour over the remainder of the body, and general lethargy).

By reconstructing ancestral states in published phylogenetic analyses of the Lake Tanganyika Synodontis and statistically comparing colour pattern divergence in the group, Wright was able to show that the aposematic coloration of the Lake Tanganyika Synodontis catfishes was derived from a single similarly-patterned common ancestor.

For more information, see the paper: Wright, JJ (2010) Conservative coevolution of Müllerian mimicry in a group of Rift Lake catfish. Evolution doi:10.1111/j.1558-5646.2010.01149.